Need your fix for more OpenGL tutorials? Unfortunatley aside from the OpenGL book all of the resources here are in C++. But cover a wide range of concepts that are definitely worth having as a reference.
OpenGL Book
Link - Concept based tutorial covering the basics of using OpenGL. Covers drawing a triangle, compiling shaders, and drawing a cube. Uses all C language, GLEW, and Freeglut. All of the fundamentals are included in this tutorial, definitely recomend takign notes.
Learn OpenGL
Link - Learn OpenGL uses C++, but it's still a bookmark work having. It's one of the most complete tutorial sites for OpenGL on the internet covering a wide range of topics including lighting and loading models.
Lazy Foo
Link - LazyFoo has been one of the longest available tutorials for OpenGL on the internet. His site uses C++, but also covers a wide range of programming concepts related to game programming. And also does so using the SDL library.
Open.GL
Link - Open.GL is another site that covers the basics, but provides tons of example code, figures and even animations to do so. The code itself is C++, but the articles are so well structured and written that it's definitely deserving of a read and a bookmark.
Songho
Link - If you want concepts, then this site has all of the concepts. From the rendering pipeline to how camera's work in OpenGL. There's not a lot of code, mostly figures and a lot of text describing how everything works.
Freeglut Wikibook
Link - One of the best tutorials for practical examples. The basic series is easy to set up and get started. Unfortunately this tutorial series also uses C++, but only for matrix manipulation. But you can find my C port of this tutorial series on Github. In fact porting the matrix manuiulation code from this tutorial from GLM to C was the birth place of DashGL, so worth taking at look at!